TIMING AND MECHANISMS OF TERMINATION OF THE INTERIOR OF A SHALLOW MARINE CARBONATE PLATFORM: THE PINGGUO PLATFORM, NANPANJIANG BASIN, GUANGXI PROVINCE, SOUTH CHINA

The Pingguo platform is one of several isolated Triassic platforms in the Nanpanjiang Basin, a rapidly subsiding embayment in the Yangtze microcontinent of the equatorial eastern Tethys. The Pingguo platform evolved from a low-relief bank followed by vast areas of platform interior termination (step back) and development of smaller pinnacle platforms. The vertical facies pattern across the termination horizon consists of: 1) molluscan dolowackestone-packstone with microbial laminite and fenestrae, 2) an interval with numerous mineralized and bored hardgrounds, 3) thick-bedded, dark-gray molluscan and oncolitic wackestone-mudstone, followed by 4) dark, organic-rich, nodular-bedded, and laminated lime mudstones, and 5) shale. Deepening and "stalling" of carbonate productivity are indicated by the hardgrounds of the molluscan and oncolitic wackestone-mudstone which may have been deposited between normal and storm wave-base. Shifting to deep, sub-photic conditions, shut down of carbonate productivity, and termination of the platform interior are indicated by the sedimentary structures and pelagic fossils in the nodular-bedded, and laminated lime mudstone. Nodular-bedded and laminated lime mudstones of the termination interval are relatively argillaceous resulting from shut down in carbonate productivity and concentration of background siliciclastics. Termination of the interior occurred during the Early Anisian (Aegean) as indicated by the conodonts Neospathodus abruptus and Neogondolella regale. A volcanic tuff horizon 85m below the termination horizon was dated at 247my using single zircon U-Pb geochronology. The age of the tuff is the same as a tuff horizon at the Scythian-Anisian biostratigraphic boundary section of another isolated platform in the northern part of the basin. Comparison of timing of termination of the Pingguo platform with other platforms in the basin demonstrates a south to north increase in platform longevity with earlier termination of the southernmost platforms. This is explained by higher rates and earlier onset of foreland basin subsidence in the southern area of the basin.